The present invention relates to an optical waveguide suitable for information transmission between internal portions of a semiconductor integrated circuit (an IC) and an optoelectronic-integrated circuit (an OE-IC) and between the ICs and the OE-ICs.
A device including semiconductor integrated circuits and optoelectronic-integrated circuits is, in general, constructed in such a way that semiconductor devices and optical devices are integrated on a semiconductor substrate and lines made of a metallic thin film are distributed between those devices. Out of them, the semiconductor devices and the optical devices perform functions such as a signal amplification, light emission and light reception, and the wiring is used to connect/transmit signals output from those devices to the associated devices.
Analyzing the speed components of the signal transmission in the integrated circuit which is manufactured by the present high-advanced technology, the speed of the signal which is propagated through the wiring area connecting the semiconductor devices and the optical devices with each other is remarkably slower than the signal speed (it is also referred to as the switching speed) of the signal which is processed in the semiconductor devices and the optical devices. For example, in the case of a high speed bipolar LSI (large scale integrated circuit) which is manufactured by the 0.8 .mu.m-processing technology, analyzing the signal speed with respect to the typical logical circuit, about 70% of the speed delay is due to the metallic wiring. Therefore, reducing the signal delay associated with the metallic wiring is required for the key technology for manufacturing such a high speed LSI.
On the other hand, an example in which the light is applied to the signal propagation and the high speed wiring is performed at the light velocity corresponds to the so-called OE-IC. FIG. 2 shows structure of an optical waveguide used in the conventional OE-IC and the like. In the figure, an optical waveguide is formed by depositing an area 3 as a wiring portion, through which the light is propagated, on a substrate 1 such as a semiconductor substrate. The area 3 has a thickness and width which are in cross section several .mu.m and several tens .mu.m, respectively, and thus the transverse size (the width) is larger than the thickness. With this structure, if in order to increase the integration, the width of the optical waveguide is made smaller to have the size near the wavelength of the light, the incidence of the optical signal becomes impossible and also the light is diffused to the periphery during the propagation thereof through the optical waveguide so that the optical waveguide can not serve as the optical signal propagating line. In addition, an example of the optical waveguide in which the transverse size is smaller than the thickness is shown in JP-A-62-143004 or JP-A-3-144514 for example. In those examples, the transverse size of the optical waveguide depends on the processing size of a trench. Therefore, if in order to increase the integration, the width of the optical waveguide is made smaller to have the size near the wavelength of the light, the incidence of the optical signal becomes impossible due to the small dispersion in the processing size, and also the light is diffused to the periphery during the propagation thereof through the optical waveguide so that the optical waveguide can not serve as the signal propagating line.
At present, as the high integration has been required year by year in order to attain the high capability of the LSI, both the width of the metallic wiring and a distance between the metallic lines need to be made finer. There is a disadvantage that if the width of the metallic wiring and the distance between the metallic lines are made finer, both the resistance value and the capacitance value per unit length of the metallic wiring increase, the signal transmission speed is increased all the more. Therefore, it is proposed that in the signal propagating line as described above, an optical waveguide is used. In addition thereto, an example relating to the prior art is disclosed in JP-A-3-223801.